Simulation of Dynamo Action Generated by a Precession Driven Flow.
Abstract
Since many years precession is regarded as an alternative flow driving
mechanism that may account, e.g., for remarkable features of the ancient
lunar magnetic field [Dwyer 2011; Noir 2013; Weiss 2014] or as a
complementary power source for the geodynamo [Malkus 1968; Vanyo
1991]. Precessional forcing is also of great interest from the
experimental point of view because it represents a natural forcing
mechanism that allows an efficient driving of conducting fluid flows on
the laboratory scale without making use of propellers or pumps. Within
the project DRESDYN (DREsden Sodium facility for DYNamo and
thermohydraulic studies) a dynamo experiment is under development at
Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in which a precession driven
flow of liquid sodium with a magnetic Reynolds number of up to Rm=700
will be used to drive dynamo action. Our present study addresses
preparative numerical simulations and flow measurements at a small model
experiment running with water. The resulting flow pattern and amplitude
provide the essential ingredients for kinematic dynamo models that are
used to estimate whether the particular flow is able to drive a dynamo.
In the strongly non-linear regime the flow essentially consists of
standing inertial waves. Most remarkable feature is the occurrence of a
resonant-like axisymmetric mode which emerges around a precession ratio
of Ωp/Ωc = 0.1 on top of the directly forced re-circulation flow. The
combination of this axisymmetric mode and the forced m=1 Kelvin mode is
indeed capable of driving a dynamo at a critical magnetic Reynolds
number of Rmc=430 which is well within the range achievable in the
experiment. However, the occurrence of the axisymmetric mode slightly
depends on the absolute rotation rate of the cylinder and future
experiments are required to indicate whether it persists at the
extremely large Re that will be obtained in the large scale sodium
experiment.